Elsevier

Applied Energy

Volume 76, Issue 4, December 2003, Pages 305-319
Applied Energy

Integrated PV and gas-turbine system for satisfying peak-demands

https://doi.org/10.1016/S0306-2619(03)00010-2Get rights and content

Abstract

A computer-simulation model of the behaviour of a photovoltaic (PV) gas-turbine hybrid system, with a compressed-air store, is developed in order to evaluate its performance as well as predict the total energy-conversion efficiency and the incurred costs under various operating conditions. This integrated PV and gas-turbine hybrid plant produces approximately 140% more power per unit of fuel consumed compared with corresponding conventional gas-turbine plants. In addition, lower rates of pollutant emissions to the atmosphere per kWh of electricity generated are achieved.

Introduction

Stored energy, e.g. in batteries, elevated water-reservoirs, super-conducting magnets or flywheels, can be recovered for use during periods of energy shortage and/or peak demand. This increases the reliability of the power supply and reduces the need for the less cost-efficient peaking units. Generally, electric utilities require a peak-load plant to be readily available as a stand-by for rapid and simple operation. At present, diesel engines, traditional gas-turbines (GTs) and pumped hydropower storage (PHPS) schemes are used to meet peak demands. PHPS, which is a potential-energy storage system, represents the most economic artificial means presently available to store energy for stimulating electricity-generating utilities. However, PHPS plants are highly expensive to install, as well as require suitable sites and long lead-times for construction [1]. A compressed-air store (CAES) provides a potential energy reservoir, via which electrical energy in excess of the demand is used to compress air, which is stored for later use to drive a turbine to generate electricity. CAES possesses the common advantages of a peak-load gas-turbine power plant and of a pumped-storage scheme. Therefore, such plants are being recognised as technically feasible and economically attractive options for load management [2], [3]. A CAES system usually has a relatively (i) high efficiency, (ii) low capital cost as well as (iii) large energy-density compared with those using other technologies [1], [3], [4].

At present, many countries around the world, e.g. Jordan, use conventional GTs to meet any shortages in available electricity-supplies occurring during an emergency or during peak-load demand periods. Such systems, for this purpose, especially those operating in an open or simple cycle, have the disadvantage of being of low efficiency and so the unit cost of generated electricity is relatively high. For example, in Jordan, gas turbines used for this purpose consumed about 25×103 tonnes of diesel fuel, but supplied less than 82 GWh, i.e. only 1.2%, of electricity generated in 2001 [5]. The average efficiency of GT peaking-plants, in Jordan, over the last 5 years, was in the range 24 to 27% [6], [7]. This can be attributed to many reasons concerning the operation mode, poor maintenance, inappropriate engine-size and age. But the most important factor is that the load control for conventional gas-turbines is accomplished by an adjustment of the fuel's flow-rate into the combustion chamber, while the airflow remains fixed. Such adjustments result in excessive rates of fuel consumption, especially with the GT under partial loads when the TIT is decreased [8]. However, the integrated CAES gas-turbine has its airflow adjusted according to the power-generation requirement, while the TIT remains constant. Thus, the specific fuel-consumption during a partial-load operation is significantly reduced [4]. Hence, the final unit cost of the produced electricity will be lower.

Also the gas emissions (that would otherwise arise from conventional generators) could be reduced by employing a hybrid system that uses a renewable energy source, such as solar energy. An example of a solar-fuel hybrid system is the integration of a solar air-heater unit with a gas-turbine power generator [9], [10]. In this solar-fuel hybrid system, a solar central-receiver plant heats the compressed air, which is then directed to a combustor. It was found that, by using such a technique, the compressed-air's temperature could be doubled and the plant's overall efficiency increased significantly. Another type of solar-fuel hybrid system is the one that uses PV panels to generate electricity in parallel with conventional generators. Many studies have been conducted to analyse the performance and feasibility of existing PV hybrid systems. It was shown that PV hybrid systems are practical and economic alternatives for many applications [11], [12], [13]. Incorporation of a PV array reduces the rate of fuel consumption, maintenance costs, and adverse impacts upon the environment. Although, for the same electrical output, the initial cost of the integrated PV system with the gas turbine is typically higher than the capital cost of conventional commensurate-power output generators, the life-cycle cost of the former can be significantly less expensive, particularly when environmental impacts are considered.

The aim of the present study is to investigate the performance of an integrated photovoltaic-gas turbine (PVGT) plant, with a compressed-air store. The PV electric generator drives an air compressor instead of consuming some of the produced shaft-power; the compressor being directly coupled with the turbine. A computer program was developed for predicting the performance of three cases, namely:

  • basic system, i.e. a conventional GT;

  • standard CAES system; and

  • proposed PVGT system.

It is not the intention of this investigation to address other associated critical issues relating to the off-design operation, compressor or turbine design and PV technology: rather guidance information concerning the proposed hybrid system is deduced, so that its overall performance can be compared with those of other basic peak-demand-satisfying systems.

This system—see Fig. 1—as in CAES, has two separate operational modes, namely compression, i.e. charging the reservoir, and expansion, i.e. discharging the reservoir. In the storage-charging mode, the motor/generator drives the compressor, which delivers high-pressure air into the reservoir. The latter may be above ground, e.g. a vessel, or underground, e.g. a cavern or porous media. In this mode, the expander is disconnected and the plant consumes power from the PV panel. In the generation mode, when electricity is required by the grid, the expander is connected by applying the clutch to the motor/generator, which acts as a generator to produce electricity.

A combination of electricity generation by means of (i) a PV panel, which is used to drive the air compressor and (ii) a gas turbine (especially in regions where conventional fuels are expensive or rarely available) is expected to improve the prospects for harnessing clean solar, energy in a more economic way. This new approach involves the use of the following main components:

  • gas-turbine engine,

  • PV array and DC/AC inverter, and

  • reservoir to store the compressed air.

The integration of the operations of these components can reduce the heat rate (HR) of the generation system significantly. This would reduce the cost of the generated electricity compared with what can be achieved with stand-alone conventional gas-turbines. In addition, lower rates of polluting emissions, per unit of electricity produced, would be released to the environment.

This newly proposed generation system is more likely to be suitable for dry and arid regions, where fresh water is scarce. The PV panel will convert the solar energy into electricity. The latter is used to power the motor/generator, which drives the compressor in the charging mode, i.e. during daytime. When electricity is required by the grid during a period of peak demand, the compressed air is allowed to flow into the combustor, where liquid or gaseous fuel is ignited to raise the temperature up to the design point. Then, the high-pressure, high-temperature gases expand through the turbine, which is connected by applying the clutch to the motor/generator. The latter, in this mode, acts as the generator and produces electricity.

In their continuous planning for load growth, the managers of electricity utilities search continually for the most economic generation schemes. But the choice will be subject to several constraints, such as the type of fuel available, peak-to-base demand ratio and the need to comply with national environmental standards. To assess the behaviour of a power plant over its expected ranges of operation, appropriate mathematical models, which can predict the performance under both design-point and off-design or part-load operating conditions have been developed. In this investigation, the performances of various peaking units are discussed. For the two systems using compressed-air storage, the main operating variable is the compressor's pressure-ratio (PR), which indicates the pressure in the reservoir, while the TIT and TET remain invariant. The insolation intensity is another variable for the PV computation. The charging-discharging time ratio, which is the air mass flow-rate during the generation mode corresponding to a specified rate during compression, is constant.

Because the CAES system is a dual-purpose plant, i.e. for energy storage and peak-power generation, having two sources of energy inputs during the charging and discharging phases, which are of different qualities, the adopted criterion for evaluating such plants differs from that usually used for a conventional GT [3], [4], [8]. More specifically, the specific fuel-consumption or HR cannot identify the thermodynamic merits of the CAES plant as in standard GT power-stations. Hence the primary-energy efficiency has been adopted as the criterion.

The thermal-power analysis of the performance of the PVGT plant was performed taking into consideration the compressor's and turbine's efficiencies, as well as the energy conversion efficiency of the motor-generator. Pressure losses in the compressor intake, reservoir piping, combustor and the turbine's exhaust-ducts were also considered as well as the variations of the specific heats of air and combustion products. These calculations were carried out by means of a bespoke tailored computer program based on mathematical models for the energy and flow matchings of the turbomachinery components, which are aerodynamically coupled along the flow to satisfy mass continuity: they are also physically coupled by the engine shaft, so that an energy balance must exist. The main points of the calculation procedure for the proposed system—see Fig. 1—are now described.

Various types of PV power-generation systems are currently operating and many papers have been published concerning their performances [14], [15], [16], [17]. Other researchers studied the long-term efficiency profile under actual insolations of a solar-tracking flat PV panel [18]. They presented the monthly energy-harnessing, the system's capacity factor, and the field efficiency for a year of the plant's operation. From the investigation of Jennings and Milne [18], an empirical relationship between the daily average radiation and the overall efficiency of the PV system was developed, namelyηPV=0.0435LnIr−0.004

Different techniques for achieving solar-radiation concentration can be used in order to reduce the capital cost of the PV power-generation system. By using optical concentrators to focus sunlight onto the PV array, its cost per kWh output can be reduced. Although a more expensive solar cell is used, it can still be cost effective due to its higher efficiency [11], [19]. One of the most adaptable concentration methods is the single-axis tracking parabolic reflector-trough [19], [20]. In this system, most of the solar cells are replaced by a glass mirror, which costs far less than the flat PV panel. The few PV cells required in the system, and located at the focal line of the trough, represent a relatively small part of the total system's cost. This means that expensive but more efficient cells can be used without incurring an economic penalty. However, the PV-trough system is still not feasible in some applications due its high capital cost. The other promising concentration technology is the point-focus procedure, which uses a reflective solar-tracking dish or Fresnel lens of high-efficiency silicon concentrator PV cells [19]. As shown in Table 1, the expected capital and O&M costs of the point-focus system can be significantly below those of other PV systems of similar kWh output and can compete with those incurred by conventional GTs in the long run.

A standard compressor is used, without inter- or after-coolers. The basic assumption is that the energy consumed by the compressor during off-peak operation equals the energy generated by the PV array during the day. Then,Wchr=IrηPVAPV

From Eq. (2), the required area (APV) of the PV array for a stipulated harnessed energy per day can be evaluated.

Single-stage combustion is proposed. The mass flow rate through the combustor equals the total flow rates of the fuel and air. The heat input during the combustion of the diesel fuel equals:Qin=ṁa1+FACpgT4−T3

There is no reheat, and the polytropic efficiency is taken as constant, with respect to the specific heat, for all stages in order to simplify the calculation. The pressure of the gases after the combustor is evaluated by considering the pressure drops through the reservoir, piping and burner: thusP4=P2ΔPr+ΔPp+ΔPbIn the case of a conventional GT, the pressure drops through the air reservoir (ΔPr) and piping (ΔPp) are eliminated from Eq. (4). The turbine's exit pressure is evaluated by considering the effect of the pressure drop in the exhaust:P5=P1ΔPe

The energy ratio, ER, for the CAES plant is expressed as the amount of pumping energy stored during the off-peak period divided by the energy generated during the peak demand period. Thus, it has been used in addition to Egen, which equals the net electric-power output, and the primary efficiency as the main indicators for the evaluation of the performance of such systems.ER=EmEgenEgenGT=Wt−Wcηgm

In the case of the PVGT system, the PV unit provides electricity to run the compressor: therefore during peak hours, all the turbine work is considered as a net output:EgenPVGT=Wtηgm

Then the heat rate (HR) is evaluated using the basic definition:HR=Qin×3600Egen

The unit price of electricity produced, and the annual expenses of the conventional GT and PVGT are evaluated respectively from the following equations:Ceu=Com+CfEgenCGT=ComGT+CfC(PVGT)=ComPVGT+Cf−Ces

For peak-load applications, medium-size GTs are commonly employed, whereas for base-load generation, heavy-duty engines are used. But for emergency electric-power generation, smaller units are usually assigned to satisfy the required emergency-demand. In this study, the selected engine, a GT 6001 B (PG 6541-B) from general electric power systems, with a nominal rating of 40 MWe, is an open cycle and single-shaft gas turbine. This engine is owned and operated, at present as standby and peaking unit in Jordan, by CEGCO and fired by diesel fuel. In order to undertake a design-point analysis for the chosen proposed plant, the practical data summarised in Table 1 were used. Such data were taken from the technical manuals of CEGCO and General Electric [21], [22].

Section snippets

Results and discussion

Jordan lies in a region of high solar-irradiation. The annual average insolation is between 6 and 8 kWh/m2-day, which is amongst the highest in the world. The total annual radiation period is about 3000 h, with an average daily value slightly exceeding 8 h [23]. This provides adequate energy for solar thermal and electrical applications, such as the proposed PVGT system.

Based on the solar radiation data [24] and the studied gas-turbine engine, the required PV area was calculated for the whole

Conclusion

Electricity utilities usually require that peak-demand plants be available promptly for operation with simple controls and low maintenance costs. At present, conventional gas-turbine and pumped hydropower storage plants are widely used for this purpose. However, compressed-air storage systems are recognized as being technically feasible and financially attractive for the supply-side, load management. This increases the reliability of the power supply and reduces the need for the less

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